This invention relates to a process for making thin, flat, dense membranes on porous substrates.
Ionic and mixed-conducting ceramic devices such as solid-oxide fuel cells, gas separation membranes, and membrane reactors require a dense electrolyte and highly porous electrodes. The motivation to fabricate thin film ceramic electrolytes derives from the benefits associated with lowering of ohmic losses across ionic and mixed ionic-electronic conducting materials as membrane thickness is reduced. When films are very thin (5-15 xcexcm) the resistance of the electrolyte at intermediate temperatures is very low, for example a xcx9c10 xcexcm thick YSZ electrolyte has been shown to have an iR drop of 0.025 xcexa9cm2 at 800xc2x0 C. This allows the device to operate at lower temperatures where less expensive materials may be used in device construction and the devices operate at higher thermodynamic efficiency. The technical challenge involves depositing a pinhole and crack free dense layer of electrolyte 2 to 50 xcexcm thickness on substrates of high porosity. The film must be well bonded to the substrate without excessive infiltration into the electrode porosity and there must be minimal interface polarization.
Several approaches to thin film fabrication have been reported including physical vapor deposition techniques, tape calendaring, sol-gel deposition, sputtering and colloidal deposition. Many of these approaches have allowed the fabrication of good quality films, however, the high cost of capital equipment and/or operating costs for several of these approaches presents a considerable barrier to their commercialization. Colloidal deposition techniques have been developed and described in the literature.
In the prior art, it was considered important to match the shrinkage rates of film and substrate. Shrinkage rates of film and substrate powders were measured by dilatometry. Although the match of shrinkage rates are still considered important, substantial improvements have been made through the discovery of the importance of matching total shrinkage of film and substrate which is a function of both shrinkage rate and green substrate density. In addition, improvements have been made in the deposition of very homogeneous thin films. In prior work, films were applied to porous substrates by inverse slip casting where porous substrates were either dipped into the YSZ suspension or the suspension was applied with a pipette to the surface of the substrate or dipping the substrate into a solution. In some cases this procedure led to pinhole free, dense, electrolyte membranes on porous substrates. A number of these structures also performed well electrochemically. However, the procedure also led to inhomogeneities in the film (differential densification) as well as different shrinkage rates for film and substrate. Quite often the bilayers were severely warped, and could only be made flat by use of substantial mass (≈100 grams/cm2) on the thin-film structure while sintering. This often led to significant edge curl, which poses significant or insurmountable problems in terms of ultimate use of these structures in solid oxide fuel cell (SOFC) stacks. Furthermore, application of the film by dip coating or application of wet dispersion often led to mud cracking and/or variations in film thickness and local green density of the film across the substrate surface.
Colloidal deposition of dense electrolyte layers on porous substrates requires that the materials be chemically compatible at the processing temperature and there must be an adequate thermal expansion match between the layers. The sintering behavior of both film and substrate materials needs to be considered. Once compatible materials have been selected, fabricating dense films of 2-50 xcexcm is achieved by careful control of the sintering profile (shrinkage vs. temperature) and the magnitude of the shrinkage of the materials. This is accomplished by systematically modifying the sintering profiles of film and substrate through control of particle size and morphology of green substrates. Poor understanding of these parameters often leads to electrolyte films of low density (pinholes) or cracked films composed of islands of high density film. Even in cases where the shrinkage of film and substrate are sufficiently close to generate dense electrolyte films, residual stresses can lead to highly distorted films with significant curling. Importantly, the electrode substrate must be processed to yield continuous porosity and a high surface area microstructure, without compromising the strength of the bilayer.
It is a general object of the present invention to provide a method of thin, dense colloidal deposition and sintering whereby high quality films of a wide variety of ionic and mixed ionic-electronic conductors can be deposited onto highly porous electrode substrates.
It is another object of the present invention to provide a method of forming thin, dense films on a substrate in which the sintering profiles of films and substrates are matched to the extent that bilayers can be free sintered to a high degree of flatness with no compressive load (or with minimal load).
It is a further object of the present invention to provide a highly flexible process in which a wide variety of materials can be deposited as thin films with no (or minimal) alterations to fabrication equipment.
It is a further object of the present invention to provide a process in which only small amounts of material are needed for bilayer fabrication making the process suitable for novel or expensive conductors.
The foregoing and other objects of the present invention are achieved by first forming a homogeneous porous substrate having a predetermined green density, aerosol spraying a suspension of homogeneous film on the surface of said substrate, firing the bilayer to sinter the film, said green substrate density being selected so that the total shrinkage of the fired film and green substrate during sintering is such that the film shrinkage is equal to or less than that of the green substrate.